This invention relates to a video signal transmitting system for a facsimile communication system, in which video signals read by scanning are transmitted.
In a facsimile communication system, an image is read by sequential scanning (or raster scanning) to be transmitted as video signals in a line format. Therefore, in the system, the recording speed at the signal receiving side is one of the factors which determine the transmission time.
Consider first the case where the facsimile device on the signal receiving side records images according to a thermal recording system. In the thermal recording system, it is necessary to provide a relatively large capacity power source for driving the thermal head. Therefore, in a facsimile device having a relatively small capacity power source, the thermal head is driven in divisional manner according to a rate B.sub.R of printing bits (hereinafter referred to as "black bits", when applicable) included in the video signals of a line (hereinafter referred to as "a black rate B.sub.R ", when applicable). In such a facsimile, the respective values of black rate B.sub.R, the numbers of division for driving the thermal head and the recording times for a line are correlated as indicated in Table 1. The facsimile, in this case, employs a blank line skipping system in which the recording operation is omitted for lines which include no black bits. Therefore, when the black rate B.sub.R is 0%, no data is indicated for the number of division in Table 1.
TABLE 1 ______________________________________ Recording Black Number of time per line rate (%) division (msec/line) ______________________________________ B.sub.R = 0 -- 2.5 0 &lt; B.sub.R .ltoreq. 25 1 5.0 25 &lt; B.sub.R .ltoreq. 100 4 10.0 ______________________________________
In transmitting video signals to the facsimile on the signal receiving side, heretofore the maximum recording time for a line is employed as the minimum transmission time. For instance in the facsimile of Table 1, 10 msec is employed as the minimum transmission time for transmitting the video signals of each line. This is to avoid the problem where the signal transmitting side transmits video signals before the signal receiving side starts the recording operation. Accordingly, in such a system, when the transmission time of the video signals of a line is shorter than the minimum transmission time, a train of fill bits is inserted in the blank part of the transmission time in transmitting the video signals. However, the system is deficient in that, when the recording time for a line is short, the black rate being low, the transmission time cannot be decreased, i.e., the efficiency of transmission of video signals is low.